The present invention relates generally to a variable attenuator having a gain set variably by a gain control circuit, and more particularly to a variable attenuator suitable for use in a radio device such as a mobile radio transceiver.
With recent development of personal use of information, there is an increasing demand for radio devices such as mobile handy-phone equipment and cellular phones. With an increase in the number of subscribers of mobile handy-phone equipment, etc., the frequencies handled by radio devices become higher in order to provide a greater number of radio channels. For example, in the PHS (Personal Handy-phone System) in Japan, 1.9 GHz is used as a carrier frequency. On the other hand, with developments of high-speed circuit techniques and mounting techniques, radio terminals capable of handling high-speed signals has been reduced in size.
However, a mobile radio terminal needs to be battery-powered. In order to reduce power consumption in the radio terminal, the control of transmission power must be controlled with high precision. A higher level of techniques has been required to achieve high-precision control of transmission power.
In the context of this description, a variable attenuator is used as an element for controlling a transmission power of, for example, the mobile radio transceiver as mentioned above. A high-precision gain control has been desired to reduce power consumption of the mobile radio transceiver.
In a general radio terminal, a baseband signal such as a voice signal, which has been processed by a baseband signal generator, is supplied to a modulator. In the modulator, a first local signal f1 from a first local signal oscillator is modulated on the basis of the baseband signal. In an up converter, provided in a subsequent stage, the modulated signal is subjected to a frequency conversion to obtain an RF signal on the basis of a frequency signal f2 from a second local signal oscillator. The RF signal is gain-controlled by a variable attenuator and then amplified to a predetermined transmission level by a power amplifier (PA). The amplified signal is output from an antenna as electric wave.
The variable attenuator is designed to set a transmission power at a desired value. The transmission power can be optimized by the high-precision gain control carried out by the variable attenuator. Thus, the high-precision gain control contributes to reducing power consumption of a transmission unit, and accordingly, the radio transceiver itself. In particular, a variable attenuator capable of performing high-precision gain control plays an important role in battery-powered mobile radio devices.
In the prior art, such a variable attenuator generally comprises .pi.-type or T-type ladder resistors and GaAs MESFETs for switching for separating/short-circuiting those resistors. In general, however, GaAs MESFET techniques are unsuitable for mobile terminals, which need to be reduced in power consumption and cost, since the price of wafers for GaAs MESFETs is about ten times higher than that of silicon wafers.
The same function as with the above-described variable attenuator can be achieved by a circuit using less expensive silicon bipolar transistors. In this circuit, input signals are supplied as differential voltage signals Vin+ and Vin-. These input signals are converted to a differential current signal by a differential amplifier. In consideration of a transistor constituting a power supply, this circuit can be constructed by cascade-connected three-stage transistors, thus permitting low-voltage operation. However, since the gain is a function of differential control signals Vcon+ and Vcon-, high-precision gain control cannot be achieved unless precise analog signals are provided to the signals Vcon+ and Vcon-.
As has been described above, in the conventional variable attenuator using bipolar transistors, the gain is determined by the function of analog control signals, and thus high-precision gain control is difficult to achieve.